This invention relates generally to a multichannel record disc recording system and more particularly to a system wherein automatic gain control is effectively carried out for each channel to be recorded. Then, the signal of each channel is matrixed and recorded on a multichannel record disc.
A previously proposed recording and/or reproducing system, for four channel record discs, is disclosed in U.S. Pat. No. 3,686,471, issued Aug. 22, 1972, and entitled "System for Recording and/or Reproducing Four Channel Signals on a Record Disc." This system is already a commercial success on a worldwide basis.
By this proposed system, sum and difference signals are formed respectively from two pairs of two channel signals, comprising the four channels. More specifically, the four signals of the first through fourth channels are respectively denoted by the notations CH1, CH2, CH3, and CH4. Sum signals (CH1 + CH2) and (CH3 + CH4) and difference signals (CHI - CH2) and (CH3 - CH4) are formed from these four signals.
Thereafter, the difference signals are angle modulated, to form angle modulated wave difference signals F(CH1 - CH2) and F(CH3 - CH4) in a frequency band which is higher than the above mentioned direct wave sum signals. These signals are mixed with the direct wave sum signals (CH1 + CH2) and (CH3 + CH4). Two sets of multiplexed signals { (CH1 + CH2) + F(CH1 - CH2)} and { (CH3 + CH4) + F(CH3 - CH4)} are formed from these direct wave sum signals and angle modulated wave difference signals. These two sets are recorded on the left and right walls of a single groove, of the 45--45 system, on a record disc. The direct wave sum signals are in a frequency band from 0 to 15 KHz, while the angle modulated wave difference signals are in a frequency band from 20 KHz to 45 KHz.
In the above described recording system, an automatic gain control circuit is used. If the level of the signal of each channel is excessive, the signal level is controlled so that recording is normally carried out. This automatic gain control circuit uses a limiting amplifier capable of automatic gain control.
If two channel signals are mutually matrixed, (i.e., the first and second channel signals or the third and fourth channel signals) they are of the same phase and the same level. For example, the sum signal obtained by their matrixing is of a level which is twice the level of each channel signal. Furthermore, if the two channel signals are of mutually opposite phase and the same level, the difference signal obtained by their matrixing has a level which is twice the level of each channel signal. Here, the difference signal does not present a very great problem even when its level becomes excessive since it is angle modulated, as mentioned above. However, the sum signal is multiplexed, as a direct wave, with the angle-modulated wave and thus recorded. Therefore, when its level becomes excessive, the direct-wave sum signal imparts interference to the angle-modulated wave.
Accordingly, it has been the practice heretofore to preset the automatic gain control circuit in each of the channel signal paths so that the level of its output signal is one half of the optimum maximum level, for recording the maximum sum signal. When the channel signals are of the same phase and the same level, the level of the sum signal is twice the level of each channel signal. The level of this sum signal becomes suitable for recording, without imparting a deleterious effect upon the angle-modulated wave difference signal component.
For this reason, the levels of the channel signals are always held, as a maximum, to one half of the optimum maximum level during a recording of the sum signal. These levels are held irrespective of the mutual relationships of their phases and levels. The recording level, of the entire recording signal or the average energy level of the recording signals, is always lowered. This has been one problem associated with the above described conventional system.
One conceivable measure for overcoming this problem is to provide an automatic gain control circuit in only the sum signal path, following the matrix circuit, instead of providing an automatic gain control circuit in each channel signal path. This arrangement overcomes the above described problem of controlling the recording level of the entire recording signal or of lowering the average energy level of the recording signals. On the other hand, there is no automatic gain control with respect to the difference signal. Hence, there are the problems in which the position of the reproduced sound source varies and the degree of separation is impaired when the record is reproduced. Therefore, this measure is not practical.